branchiae

Branchiae are gill-like respiratory organs found in some aquatic invertebrates, most notably many polychaete annelids. The term branchia (plural branchiae) derives from Latin and refers to gills. Branchiae function as sites of gas exchange between the animal and the surrounding water.

In polychaetes, branchiae are outward extensions of the body wall. They vary in form from slender filaments and tufts to leaf-like plates, and may be paired on many body segments or restricted to a few. They are typically richly vascularized and kept moist to facilitate diffusion of gases. Some species bear ciliated epithelia that help move water over the gill surfaces.

Physiology: Water flow over branchial surfaces is driven by ciliary action or by movement of the body and parapodia. Oxygen dissolved in water diffuses into the blood while carbon dioxide diffuses out. Branchiae thus provide the primary respiratory surface in many polychaetes, especially in taxa with external gills and in environments where dissolved oxygen varies.

Evolution and taxonomy: The presence, absence, or modification of branchiae varies among polychaete lineages and may reflect adaptations to habitat, depth, and oxygen availability. In some groups branchiae have been reduced or lost altogether. The concept of branchiae is largely used in annelid anatomy; molluscan gills are commonly referred to as ctenidia rather than branchiae.